This invention relates to a method for preparing phosphite stabilizers and more particularly to a method for preparing bis-(dialkyphenyl) pentaerythritol diphosphites. Still more particularly, this invention relates to a process for preparing a mixture of bis-(2,4-di-t-butylphenyl) pentaerythritol diphosphite isomers having a high proportion of spiro isomer.
A great number of organo-phosphite stabilizers are known in the art and several general methods are available for their preparation. Widely used for this purpose are ester interchange reactions such as those disclosed for example in U.S. Pat. Nos. 2,961,454, 3,047,608, 3,204,250 and 3,787,537. The preparation of di-(2,4-di-t-butylphenyl) pentaerythritol diphosphite from a dialkyl or diaryl pentaerythritol diphosphite and a slight excess of 2,4-di-t-butylphenol by an ester interchange process is disclosed in U.S. Pat. No. 4,305,866. As is now well known in the art, and is set forth in the latter patent, pentaerythritol diphosphites produced by these processes are generally mixtures of the spiro and cage isomer, viz. ##STR1##
The proportion of the two isomers in the product mixture varies both with the method of preparation and with the nature of R. Transesterification processes usually provide spiro/cage ratios near 1/1 where R=alkyl, while ratios of 3/1 are more commonly produced where R=aryl. There is also some suggestion in the art, as is set forth in U.S. Pat. No. 4,290,976, that the spiro and cage isomers will be inter-converted at temperatures above about 75.degree. C. where R=alkyl, increasing the proportion of cage isomer.
The proportion of spiro and cage structures in the product mixture significantly affects the physical characteristics of the product. As is well known, pure compounds exhibit sharp melting points, while mixtures exhibit broad and usually depressed melting ranges. For most pentaerythritol diphosphites, the spiro isomer has a significantly higher melting point than the cage isomer. Increasing the proportion of spiro isomer tends therefore to raise the melting range temperature of the mixture which reduces the tendency toward having waxy, low softening temperature characteristics. Even more beneficial are methods for isolating the spiro isomer in a pure or nearly pure form, such as by selective recrystallization from a suitable solvent. Selective recrystallization, however, may result in a substantial reduction in overall yield if the cage isomer constitutes a major fraction of the mixture, thereby markedly increasing the cost of manufacture.
An economically more favorable and therefore more desirable process would be one in which the spiro isomer is the predominant product. Such a process, particularly when coupled with a selective crystallization step, could provide good quality spiro isomer in greater yields, and thus have a significant manufacturing cost advantage.